In recent years, there has been an increasing interest in the use of ceramics for structural applications historically served by metals. The impetus for this interest has been the superiority of ceramics with respect to certain properties, such as corrosion resistance, hardness, wear resistance, modulus of elasticity, and refractory capabilities when compared with metals.
However, a major limitation on the use of ceramics for such purposes is the feasibility and cost of producing the desired ceramic structures. For example, the production of ceramic boride bodies by the methods of hot-pressing, reaction sintering and reaction hot pressing is well known. In the case of hot pressing, fine powder particles of the desired boride are compacted at high temperatures and pressures. Reaction hot pressing involves, for example, compacting boron or a metal boride with a suitable metal-containing powder, also at elevated temperatures and pressures. U.S. Pat. No. 3,937,619 to E. Clougherty describes the preparation of a boride body by hot-pressing a mixture of powdered metal with a powdered diboride, and U.S. Pat. No. 4,512,946 to M. Brun describes hot-pressing ceramic powder with boron and a metal hydride to form a boride composite. However, these hot pressing methods require special handling and expensive special equipment, they are limited as to the size and shape of the ceramic part produced, and they typically involve low process productivities and high manufacturing costs.
A second major limitation on the use of ceramics for structural applications is their general lack of ductility and toughness (i.e., damage tolerance or resistance to fracture). This characteristic tends to result in sudden, catastrophic failure of ceramics in applications involving even rather moderate tensile stresses.
One approach to overcome this problem has been to attempt to use ceramics in combination with metals, for example, as cermets or metal matrix composites. The objective of this approach is to obtain a combination of the best properties of the ceramic (e.g. hardness) and the metal (e.g. ductility). A method for producing a ceramic-metal composite (cermet) is disclosed in European Application No. 0 116 809. According to this disclosure, there is first provided a bulk reaction mixture of particulate reactants, and this mixture is then reacted upon contact with molten metal which infiltrates the mixture. Exemplary of such a reaction mixture is one containing titanium dioxide, boron oxide, and aluminum (in stoichiometric amounts and in particulate form), which upon contact with molten aluminum, reacts to form titanium diboride and alumina as the ceramic phase that is infiltrated by the aluminum. It is clear from the disclosure that the molten metal, i.e. aluminum, is a reducing agent and not a precursor to a boride forming reaction.
European Application No. 0 113 249 discloses a method for making a cermet by first forming dispersed particles of a ceramic phase in situ in a molten metal phase, and then maintaining this molten condition for a time sufficient to effect formation of an intergrown ceramic network. Formation of the ceramic phase is illustrated by reacting a titanium salt with a boron salt in a molten metal. The ceramic is developed in situ and becomes an intergrown network. There is, however, no infiltration; and further the molten metal employed in the process, e.g. aluminum, is a reducing agent and does not react to form a boride, and the boride is formed as a precipitate in the molten metal. Both examples in the application expressly state that no grains were formed of TiAl.sub.3, AlB.sub.2, or AlB.sub.12, but rather TiB.sub.2 is formed demonstrating the fact that the aluminum is not the metal precursor to the boride.
U.S. Pat. No. 3,864,154 discloses a ceramic-metal system by infiltration. A compact of AlB.sub.12 was impregnated with molten aluminum under vacuum to yield a system of these components. Other materials prepared included SiB.sub.6 -Al; B-Al; B.sub.4 C-Al/Si; and AlB.sub.12 -B.Al. There is no suggestion whatsoever of a reaction, and no suggestion of making composites involving a reaction with the infiltrating metal nor of any reaction product embedding an inert filler or being part of a composite.
While these concepts for producing cermet materials have in some cases produced promising results, there is a general need for more effective and economical methods to prepare such ceramic-metal composites.